Abstract
Aims: We have investigated a frequency-dependent shift in the
absolute position of the optically thick apparent origin of parsec-scale
jets ("core shift" effect) to probe physical conditions in ultra-compact
relativistic outflows in active galactic nuclei. Methods: We used
multi-frequency Very Long Baseline Array (VLBA) observations of 191
sources carried out in 12 epochs in 2006 within the Monitoring Of Jets
in Active galactic nuclei with VLBA Experiments (MOJAVE) program. The
observations were performed at 8.1, 8.4, 12.1, and 15.4 GHz. We
implemented a method of determining the core shift vector based on (i)
image registration by two-dimensional normalized cross-correlation and
(ii) model-fitting the source brightness distribution to take into
account a non-zero core component offset from the phase center.
Results: The 15.4-8.1, 15.4-8.4, and 15.4-12.1 GHz core shift vectors
are derived for 163 sources, and have median values of 128, 125, and 88
μas, respectively, compared to the typical measured errors of 50, 51,
35 μas. The effect occurs predominantly along the jet direction, with
departures smaller than 45° from the median jet position angle in
over 80% of the cases. Despite the moderate ratio of the observed
frequencies (2σ) are detected for about 55% of the sources. These shifts
are even better aligned with the jet direction, deviating from the
latter by less than 30° in over 90% of the cases. There is an
indication that the core shift decreases with increasing redshift.
Magnetic fields in the jet at a distance of 1 parsec from the central
black hole, calculated from the obtained core shifts, are found to be
systematically stronger in quasars (median B1 ≈ 0.9 G)
than those in BL Lacs (median B1 ≈ 0.4 G). We also
constrained the absolute distance of the core from the apex of the jet
at 15 GHz as well as the magnetic field strength in the 15 GHz core
region.
Full Tables 1 and 4 are available in electronic form at http://www.aanda.org
Original language | English |
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Article number | A113 |
Number of pages | 15 |
Journal | Astronomy & Astrophysics |
Volume | 545 |
DOIs | |
Publication status | Published - 2012 |
MoE publication type | A1 Journal article-refereed |
Keywords
- galaxies: active
- galaxies: jets
- radio continuum:
- galaxies
- quasars: general